This invention relates to lens frangibility testing apparatus and more particularly to an impact hammer for testing the strength of eyeglass lenses.
The regulations of the Food and Drug Administration require that eyeglass lenses must be capable of withstanding the impact force generated by a five-eighths inch diameter steel ball weighing approximately 0.58 ounces dropped from a height of 50 inches upon the horizontal upper surface of the lens. The ball must strike within a five-eighths diameter circle at the geometric center of the lens. The ball may be guided but not restricted in its fall by being dropped through a tube extending to within approximately 4 inches of the lens. The total weight of the base plate and its rigidly attached fixtures is required to be not less than 27 pounds.
The conventional type of lens frangibility testing device is a tower-like structure having a long, verticallydisposed tube with a pedestal at the base for supporting a lens. Since the drop-ball apparatus is normally positioned on a table top, the operator must mount a ladder or some type of platform to place the ball at the top of the tube. Thus, either two persons are required to operate the apparatus or one operator quickly becomes fatigued. Danger of injury from falling presents an additional problem. Alternately, some embodiments utilize a raising device to convey the ball to the top of the device. The apparatus required for this approach is relatively complicated and involves many moving parts, thus increasing maintenance requirements. In either case, precision is a problem since the tube must be larger than the diameter of the ball and cannot extend down to the lens. Thus, the ball may strike at different points on the lens near the center rather than at the exact center, resulting in a considerable degree of inaccuracy and error.